Telomeres, the nucleoprotein structures at the ends of eukaryotic chromosomes, are maintained and protected by a suite of highly specialized proteins. These proteins are responsible for chromosome maintenance and play important roles in cancer and aging. A critical activity at telomeres is the proper capping and processing of a conserved GT-rich single-stranded overhang. Loss of the ability to discriminate this natural end from damaged DNA is catastrophic, leading to severe genomic instability. Access of the replicative enzyme telomerase to the overhang is tightly controlled, as this activity regulates the proliferative potential of the cell. One of the key hallmarks of cancer cells is their unlimited proliferative potential, which is, in turn, dependent on telomere maintenance and length regulation. Telomerase is activated in approximately 85% of tumor cells, but not in neighboring non-tumorous cells. However, premature telomere shortening can also lead to a variety of serious disease states. The goal of our work is to understand how modulation and control of the protein and DNA structure at the end of the telomeres mediates these essential activities. Our project focuses on the structures and activities of key proteins found at the telomeric single stranded overhang in the model organism budding yeast and in higher eukaryotes. In the budding yeast, the essential protein Cdc13 binds the single stranded overhang, protects the ends of chromosomes from degradation, and regulates the activity of telomerase. Each of these activities is critical for maintaining telomere integrity and function. The knowledge gained from the study of Cdc 13 will accelerate our ability to understand telomere maintenance in humans and other higher eukaryotes. This project will extend our knowledge of Cdc13 function, by examining the elements of molecular recognition involved in the interaction of Cdc13 with telomeric DNA using biochemical, biophysical, and structural methods. In addition, this project will investigate the structural and biochemical attributes of an important regulatory domain within Cdc 13 that plays a crucial role in telomere length regulation. Finally, this project will examine telomere structure in higher eukaryotes, where the recently discovered Pot1 proteins bind the single-stranded overhang and perform end protection and telomerase recruitment activities. This work will contribute to a molecular level understanding of telomere end protection and length regulation, and will provide the structural and functional knowledge needed to help design new therapeutics for cancer and aging.